This invention relates to fuel compositions which have been blended with other components which may or may not be hydrocarbons in such a manner that the resultant blend gives rise to reduced particulate emissions from the exhausts of vehicles powered by combustion of such fuels.
Fuels such as diesels are of particular interest and are used rather widely in automotive transport and for providing power for heavy duty equipment due to their high fuel economy. However, one of the problems when such fuels are burned in internal combustion engines is the pollutants in the exhaust gases that are emitted into the environment. For instance, some of the most common pollutants in diesel exhausts are nitric oxide and nitrogen dioxide (hereafter abbreviated as xe2x80x9cNOxxe2x80x9d), hydrocarbons and sulphur dioxide, and to a lesser extent carbon monoxide. In addition, diesel powered engines also generate a significant amount of particulate emission which include inter alia soot, adsorbed hydrocarbons and sulphates, which are usually formed due to the incomplete combustion of the fuel and are hence the cause of dense black smoke emitted by such engines through the exhaust. The oxides of sulphur have recently been reduced considerably by refining the fuel, e.g., by hydrode-sulphurization thereby reducing the sulphur levels in the fuel itself and hence in the exhaust emissions. The presence of particulate matter in such exhaust emissions has been a cause for concern. It is known that the cause of the particulate matter emission is incomplete combustion of the fuel and to this end attempts have been made to introduce into the fuel organic compounds which have oxygen value therein (hereafter referred to as xe2x80x9coxygenatesxe2x80x9d) to facilitate combustion. Oxygenates are known to facilitate the combustion of fuel to reduce the particulate matter and they are also ashless. However, high treat rates are required which means that these cannot be classed simply as minor additives but these become significant components of the fuel composition. Whilst the oxygenates and other components used hitherto in fuels have primarily focussed on the oxygen values and their effect of combustion of fuel, it has hitherto been unrecognized that the performance, especially in respect of reduced particulate emission, can be significantly improved by controlling the volatility of the front to mid-range components in the fuel. In other words, by depressing the temperature range within which the front to mid-range components distil, the particulate emissions from a given fuel composition can be significantly reduced.
It has now been found that this depression of the temperature range within which the front to mid-range components in the fuel are found can be achieved by blending the fuel with suitable materials which can be oxygenates or other hydrocarbon components.
Accordingly, an embodiment of the present invention is a diesel fuel composition comprising a major amount of a base fuel and a relatively minor amount of at least one chemical component other than that generated in a refinery process stream which component is miscible with the base fuel in such proportions that the T30 temperature of the resultant composition is in the range from 203-250xc2x0 C.
By xe2x80x9cT30 temperaturexe2x80x9d as used herein and throughout the specifications is meant the temperature by which 30% by volume of the fuel has distilled and is measured using the ASTM D86-95 test method.
By xe2x80x9ca chemical component other than that generated in a refinery process streamxe2x80x9d is meant a component which is not the direct product of a refining process but may be a product from a chemical plant associated with a refinery. Thus blends of fractions of a refining process are not contemplated as a xe2x80x9cchemical componentxe2x80x9d under the present invention.
The fuels that may be used in and benefit by the compositions comprise inter alia distillate fuels, and typically comprise a major amount of diesel fuel, jet fuel, kerosene or mixtures thereof. The diesel fuel used is preferably ashless. The distillate fuel itself may be obtained by conventional refinery distillate methods, or may be synthesized, e.g., by the Fischer-Tropsch method or the like. It is preferable, however, that the olefin content of the base fuel is no more than 10% by weight. The fuel is most preferably a low sulphur diesel fuel with a sulphur content of 500 ppm or less. One such low sulphur base fuel is obtainable from Esso""s Refinery at Fawley, UK.
It is also preferable that the diesel fuel compositions are substantially free of C1-C2 alcohols and thus the compositions do not embrace gasohol type compositions which contain significant amounts of ethanol and/or methanol; the present compositions contain no more than adventitious amounts of these alcohols, e.g., not more than 5% by weight of such alcohols, and preferably no C1-C2 alcohols at all.
It is known that the T30 temperature of most of the conventional diesel fuels is from about 250-280xc2x0 C. The feature of an embodiment of the invention is to blend such conventional base fuels with one or more components in such amounts that the T30 temperature of the resultant blend is within the range from 203-250xc2x0 C., suitably from 205-240xc2x0 C., preferably from 210-235xc2x0 C.
To bring the T30 temperature within the desired range, the base fuel may be blended with a variety of minor chemical components. It is preferable that the minor chemical component blended with the fuel has a boiling point which is below the desired upper limit of the T30 temperature of the resultant blend, e.g., below 240xc2x0 C. if it comprises a single entity or has a T50 below 240xc2x0 C. if it comprises a mixture of components. For instance, the base fuel may be blended with a hydrocarbon fraction from a chemical plant associated with the refinery to achieve this effect. An example of such a hydrocarbon fraction is one or more alkanes, for example a mixture consisting of primarily isodecanes. Alternatively, the minor chemical component in such a fuel may be one or more aliphatic hydroxy compounds selected from alcohols, glycols, triols, polyols and ethers alcohols; full ethers of such hydroxy compounds, partial or full esters of one or more of the hydroxy compounds with aliphatic mono-, di-, tri- or poly-carboxylic acids. The hydroxy compounds may be comprised of primary, secondary or tertiary hydroxy functions and may be straight or branched chain. The hydroxy compounds suitably have 6 to 20 carbon atoms and preferably from 8 to 16 carbon atoms. Specific examples of such hydroxy compounds include the monohydric alcohols selected from one or more of n-hexanol, methyl pentanols, n-octanol, isooctanol, n-nonanol, isononanols, n-decanol, isodecanol, n-undecanol, isoundecanol, n-dodecanol, isododecanol, tridecanol and isotridecanol. Some of these alcohols are commercially available as Exxal(copyright)10 and Exxal(copyright)12 from Exxon Chemicals. The glycols and polyols suitably have from 2 to 20 carbon atoms and these may be polyether diols or polyols. The ethers referred to above suitably contain from 5 to 20 carbon atoms. The two hydrocarbyl groups attached to the ethereal oxygen atom may be in the form of primary, secondary or tertiary alkyl groups, aryl groups and the two hydrocarbyl groups may be the same or different. Specific examples of such ethers include methyl tertiary butyl ether, ditertiary butyl ether and anisole. The esters may be derived by reacting one or more of the aliphatic carboxylic acids referred to above with the hydroxy compounds referred to above.
The amount of any of the minor chemical components referred to above blended with the base fuel to form the fuel compositions of embodiments of the present invention will depend upon the chemical characteristics of the minor chemical component. For instance, it is most desirable that the boiling point of the minor chemical component is below 240xc2x0 C. and that it is miscible with the base fuel over a wide range. Thus, if the base fuel is blended with another hydrocarbon fraction, the boiling point and degree of miscibility of this hydrocarbon fraction would be significant in determining the amount blended with the base fuel. Similarly, if an oxygenate is blended with the base fuel, the amount of oxygenate blended would be determined by the miscibility of the oxygenate with the base fuel, the number of oxygen atoms in the oxygenate and the boiling point of the oxygenate. Typically, the amount of the miscible minor chemical component blended with the base fuel is suitably at least 5% by weight of the total composition. Typically, if an oxygenate is used, it is preferably such that it brings the T30 temperature of the resultant blend within the range from 205-240xc2x0 C. Thus, to achieve this composition, the amount of oxygenate added to the composition is suitably greater than 5% by weight of the total composition, and is preferably greater than 7% w/w of the total composition. Typically, the oxygenates are used in an amount in the range from 5 to 60% by weight, preferably from 7 to 40% by weight of the total composition. Within these ranges, it would be possible to use a relatively low amount of a specific oxygenate if said oxygenate has a relatively high oxygen content and conversely, one may have to use a higher amount of a particular oxygenate if it is relatively low in oxygen content so that the blended composition has at least 0.5% w/w of oxygen, suitably at least 1.0% by weight of oxygen and preferably at least 2% by weight of oxygen.
Thus, according to a further embodiment, the present invention is a method of reducing particulate emissions upon combustion of a diesel fuel composition, said method comprising blending the base diesel fuel with a minor amount of a miscible chemical component other than that generated in a refinery process stream in sufficient amount such that the T30 temperature of the resultant composition is within the range from 203-250xc2x0 C.
The blending may be carried out by conventional means of intimate mixing of the base fuel with the minor chemical component. The diesel fuel compositions having a T30 temperature within the range from 203-250xc2x0 C., preferably from 205-240xc2x0 C. of the present invention are capable of reducing particulate emissions both at high and low loads.
The diesel fuel compositions the present invention and their performance are further illustrated with reference to the following Examples and Comparative Tests: